Description of the Prior Art
In the manufacture of plastic filling materials for teeth it is common practice to utilize powdered glass or quartz as an inert filler for the resin binders utilized.
The refractive index of fillers and the refractive index of cured conventional acrylic binders used in dental and medical restorative compositions are similar and ordinarily on the order of 1.45 to 1.56. Upon placement of such silica glass-filled plastic restorative and cure of the acrylic resin binder, a translucent restoration is achieved. Such restorations reflect color from the adjacent tooth surfaces and blend nicely with the shade of the restored tooth particularly when the restoration is not too large.
Conventional silica glass-containing restorations lack X-ray opacity requisite to permit the dentist to distinguish the restorative material from decalcified tooth structure such as is present in a carious lesion and to detect subgingival overhangs. To provide X-ray opacity, barium glass is sometimes utilized in lieu of conventional glass in such restorative compositions. Barium glass having a refractive index in the desired range of 1.5 to 1.6 is, however, characterized by limited X-ray opacity and is often unduly soft and soluble in strong acids, hence plastic restorations utilizing such barium glass do not reliably afford a long service life and are of relatively low compressive strength. For example, conventional acrylic restorative compositions filled with odinary silica or borosilicate glass may achieve a compressive strength of about 45,000 psi whereas barium glass restorative compositions containing barium of refractive index in the range of about 1.5 to 1.6 seldom if ever achieve compressive strength values in excess of 30,000 psi.